SUMMARY: Loopback filesystem and SUID

From: Jesús Cea Avión (
Date: Tue Aug 25 1998 - 15:35:52 CDT

Original Question at the end.

Thanks to:

Casper Dik <casper@holland.Sun.COM> (Kevin Sheehan {Consulting Poster Child})

A "man lofs" states *clearly* that except "read only", all atributtes
are inherited from the underlying filesystem. So the lofs can´t be
"no-SUID" if the underlying filesystem is "SUID-enabled".

Casper also says (and I tried it :) that you can't erase or create new
files, but it's perfectly possible to modify the existing ones:

  "Lofs doesn't create new vnodes for the loopbackl files, only for
   the directories (efficiency reasons).

   At the per-file basis, "ro" and "nosuid" cannot be implemented as
   you'll only see the files from the underlying filesystem."

In this way, "lofs" utility is severely decreased :(. That's my

Casper suggest, nevertheless, to use NFS partitions in order to mount
them as Read-Only + NOSUID. A valid suggestion if you can mount them two
times on different directories; one as RW and the other as RO...

With these restrictions, "lofs" is useful, yet, if CHROOT security is
enough for you, and you use "lofs" *ONLY* to avoid file replication
inside the CHROOT environmment.

> I'm trying to mount a lookback filesystem in order to improve my site
> security (lofs+chroot). I can do a "mount -r dir_to_mount
> new_location" to mount it as Read-Only.
> Nevertheless I can´t find the way to mount the lofs as "noSUID". At
> least no if the underlying partition is "SUID enabled". Anybody can
> help me?. And yes, the underlying partition must be mounted as
> read-write and SUID. The loopback can be set to read-only but I've
> can't make it no-SUID.
> I'll sumarize.
> PS: lofs=Loopback Filesystem

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